The very first responder to a Florida high school shooting that killed 17 people said Monday he never entered the building to confront the suspect because he believed the gunfire was coming from outside.

Scot Peterson, the armed deputy assigned to Marjory Stoneman Douglas High School, has been called a coward and worse for failing to stop the shooting.

In his first public statement about the shootings, Peterson said it's "patently untrue" that he failed to meet department standards or acted with cowardice as the first armed responder on the scene of the Valentine's Day massacre.

Faced with possible termination after Broward County Sheriff Scott Israel said he felt "sick to my stomach" over his deputy's failure to intervene, Peterson resigned and indicated his immediate retirement.

"Let there be no mistake, Mr. Peterson wishes that he could have prevented the untimely passing of the seventeen victims on that day, and his heart goes out to the families of the victims in their time of need," his attorney, Joseph DiRuzzo, said in the statement.

The lawyer said Israel's account of Peterson's actions that day were a "gross oversimplification" of the events.

Politicians and other officials have been withering in their criticism. President Donald Trump blamed not only Peterson but other officers who didn't stop the attack, saying Monday that they "weren't exactly Medal of Honor winners." Speaking to a group of governors at the White House, Trump said "I really believe I'd run in there even if I didn't have a weapon and I think most of the people in this room would have done that, too."

Peterson's statement said he and a security specialist ran to the scene at first word of the shooting, a report that mistakenly said firecrackers were being set off near the 1200 building. He then heard gunshots, "but believed that those gunshots were originating from outside of the buildings."Following his training to seek cover and assess the situation in the event of outdoor gunfire, he "took up a tactical position" between two nearby buildings while alerting dispatch that he heard gunfire, and initiating a "Code Red" lockdown of the campus.

"Radio transmissions indicated that there was a gunshot victim in the area of the football field," adding to his belief that the shooting was outside.

Florida Gov. Rick Scott's office has asked Florida Department of Law Enforcement Commissioner Rick Swearingen to investigate the law enforcement response, and the agency confirmed it would begin the probe immediately.

Meanwhile, former Miami Beach Mayor Philip Levine, now a Democratic candidate for governor, led a rally of more than 1,000 people in heavy rain on the Capitol steps in Tallahassee, calling for a ban on assault rifles and criticized the National Rifle Association for advocating to arm teachers.

"We know that is dumb, dumb, dumb!" Levin said to cheers and applause.

Bused in from around the state, protesters wore orange T-shirts saying #GunReformNow. One held a sign with an image from the movie "The Sixth Sense," with the words "I SEE DEAD PEOPLE ... THANKS TO THE GOP AND NRA." Another said "Hey Lawmakers! Take the Pledge. No NRA Money."

One tearful student who has undergone three surgeries on her wounds and still has bullet fragments inside her also spoke publicly for the first time, thanking the doctors and first responders for helping her make what she says will be a full recovery. Maddy Wilford, 17, said at a hospital news conference that "it's times like these when I know that we need to stick together."

Rescuers thought Wilford was dead when they first encountered her inside the school. She was pale and unresponsive, bleeding heavily from bullet wounds to her chest, abdomen and arm. A fire-rescue lieutenant was under orders to take her to a hospital 30 miles away, but made what doctors called a life-saving split-second decision to bring her instead to a hospital less than 10 miles away that had practiced an active shooter drill months earlier."She's very lucky, very, very lucky" said Igor Nichiporenko, M.D., the medical director of trauma services at Broward Health North, adding that the large caliber bullets "penetrated through her chest and abdomen."

Flanked by her mother and father, Wilford said that with "all the love that's been passed around, I definitely wouldn't be here without it."

On Sunday, thousands of students, many with their parents, entered the campus for the first time since the shooting, gathering their backpacks and other things abandoned as they fled. The three-story 1200 building Nikolas Cruz allegedly fired his AR-15 assault weapon before melting into the crowds of fleeing students is now cordoned off by a chain link fence, covered with banners from other schools showing their solidarity.

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Associated Press writers Freida Frisaro and Adriana Gomez Licon in Miami; Gary Fineout in Tallahassee, Florida; Tamara Lush in St. Petersburg, Florida; and Jeffrey Collins in Columbia, South Carolina; contributed to this report.